1. Field of the Invention
The present invention relates to a printer which prints a half-tone image on a recording material based on image signal entered through an image input device. The present invention relates also to an image processing method for processing the image signal to produce optimum recording data in view of recording characteristics of the printer and the recording material.
2. Background Arts
A video printer has been known, which receives image signal, e.g. analog color video signal or digital image signal, from an external device, and prints a full-color half-tone image on a recording material based on the image signal. When the image signal is analog video signal, it is converted frame by frame into digital three color (RGB) image data, or into luminance and chrominance data. The digital image data is processed into recording data for yellow, magenta and cyan. A recording head of the printer is driven in accordance with the recording data.
Since the dynamic range of the tone reproduction curve of the printer is narrower than the signal dynamic range of the video signal, without any correction, dark areas of the scene tend to be darker in the printed hard copy in comparison with the video image displayed on a screen. For the same reason, light areas of the scene tend to be lighter in the hard copy compared with the video image.
As a gradation correction method, it is known to determine an appropriate low luminance level as a black level and an appropriate high luminance level as a white level within a luminance range of the image signal, and assign luminance levels between the black level to the white levels respectively to gray levels between maximum and minimum densities available for recording, in accordance with the following gradation conversion formula: EQU D=(Dz-D0).multidot.(Da-Sb)/(Dw-Db)+D0
wherein PA1 producing a luminance histogram showing frequencies of respective luminance levels from the image signal; PA1 determining a black luminance level and a white luminance level based on the luminance histogram; PA1 assigning the black luminance level and the white luminance level respectively to a darkest tonal level and a lightest tonal level of a gradation range available for printing; PA1 multiplying the frequency of each luminance level with a different luminance coefficient predetermined according to the luminance level; PA1 accumulating multiplication results of all the luminance levels to use the accumulation result as a scene discrimination coefficient; PA1 determining a gradation conversion curve depending upon the scene discrimination coefficient; and PA1 assigning the respective luminance levels of the image signal to tonal levels ranging from the darkest tonal level to the lightest tonal level in accordance with the gradation conversion curve.
Dw is the white level; PA2 Db is the black level; PA2 Da is a luminance level of the image signal, Db.ltoreq.Da.ltoreq.Dw; PA2 Dz is a luminance level corresponding to the minimum recording density; PA2 D0 is a luminance level corresponding to the maximum recording density; and PA2 D is a luminance level corresponding to a gray level to which the luminance level Da is assigned, D0.ltoreq.D.ltoreq.Dz.
This method cannot sufficiently correct the above described problem when the luminance distribution is biased toward low luminance side, i.e. lowkey image, or high luminance side, i.e. highkey image.
In another conventional gradation conversion method, as disclosed in JPA 61-157191 for example, a luminance histogram of the image signal is detected, and in accordance with the frequency distribution of the luminance histogram, a gradation conversion curve is selected which allots more tonal steps to those luminance levels having larger frequencies. This method is known as histogram flattening.
According to the histogram flattening method, if the image includes a portion where the luminance varies gradually over a wider range, gradation smoothness of that portion would be deteriorated, because the tone changes steeply in the density range corresponding to the high frequency luminance levels, while the tone changes little outside the density range corresponding to the high frequency luminance levels. Therefore, the histogram flattening is not suitable for reproducing natural tone images.
In addition, there are differences in luminance range between input image signals according to the image input device. For example, the luminance signal level for white of the video signal can correspond to 100 IRE or 140 IRE. As for the black level, the luminance signal level varies depending upon whether the image input device has a setup function or not. The same applies to digital image data entered through a computer or the like.
The difference in the luminance range of the input image signal has an amplified effect on the tone reproduction of the hard copy. The conventional histogram flattening method can not eliminate the bad effect of the luminance range difference between the image signals.